This invention relates to a method and a device for generating wire bonds.
Methods of the aforementioned kind are particularly employed in order to electrically connect electrical contact members (also called pads) of integrated circuits, e.g. microchips, to electrical contact members of substrates of the latter circuits.
Among the most valuable methods for generating these bonds are the so-called ball/wedge bonding and the wedge/wedge bonding methods.
In ball/wedge bonding the wire that is to be bonded with electrical contact members of the microchip (which may be formed integral with the microchip) as well as those of the substrate bearing the microchip, is guided in an elongated hollow shaped tool in form of a capillary.
In a first step a free end portion of the wire protruding from the endface of the tool towards a first electrical contact member of the microchip is melted to form a ball by a hydrogen flame or a discharge of a capacitor. Thereafter the ball is positioned on the first electrical contact member and welded to it by pressing it with a first force (also called bond force) onto the surface of the first electrical contact member, wherein said bond force is normal to said surface, while causing the wire (i.e. the ball) to vibrate with ultrasonic frequency along the surface of the first electrical contact member. In addition heat may be applied to the substrate during the welding procedure (thermosonic bonding).
After having generated the bond between the formerly ball shaped end portion of the wire and the first electrical contact member, the wire is released and the tool is lifted from the surface of the first electrical contact member. Then the tool is moved above a second electrical contact member of the substrate to which the first electrical contact member is to be connected thereby forming a wire loop. Again—by means of the tool—a first surface of a portion of the wire (the wedge) is pressed downwards onto a second surface of the second electrical contact member while said portion of the wire is vibrated at the same time along the second surface with ultrasonic frequency by way of the tool which may comprise a transducer for generating the ultrasonic vibration. After having generated the bond the wire is clamped and torn away, and the next ball shaped end portion of the wire is formed and the process is repeated until the microchip is properly connected to its substrate.
In case welding a ball shaped end portion of the wire to the first electrical contact member of the microchip is substituted by welding a wedge to the microchip's first electrical contact member, the method is called wedge/wedge bonding. In this case positioning of the wedge cannot be accomplished by movements of the tool along the three spatial dimensions x, y, and z, but also needs a rotation about a vertical θ-axis since the wedges have to point in the direction of the wire loops connecting the first electrical contact members to the second electrical contact members.
Wires used in the above methods preferably have a diameter of the order 10 micrometers to 100 micrometers and are typically made from AlSi1, Al and Au. The ultrasonic vibration applied to the wire is of the order 60 kHz to 120 kHz. In principle, standard wires made from AlSi1 having a diameter of 10 to 50 micrometers and so-called thick wires made from Al having a diameter of 100 to 400 micrometer are distinguished, wherein standard wires are bonded within milliseconds and thick wires within several hundred milliseconds.
Formation of a bond evolves in three characteristical stages. In a first stage a close physical contact is established between the portion of the wire and the electrical contact member such that the interaction between the bond partners is governed by van der Waals forces, wherein impurities are transported away from the connection region due to the vibration of the wire in a plane normal to the bond force (cleaning effect), i.e., the friction breaks the impurities—these are AlO2-crystals in case of Al—from the surface, the transport is achieved by the transport power, i.e., the wires bond surface gets deformed which movement transports away the impurities. In a second stage a chemical (metallical) connection is about to form, wherein welded micro regions are distributed statistically. In a third stage diffusion processes into the bulk start. The interfacial area grows while mechanical tensions decrease. The formation of the bond is accelerated due to the mechanical effect of the ultrasonic vibration and the related heat.
In DE 44 47 073 C1 a method for generating a wire bond is described in which a first surface of a portion of the wire is pressed against a second surface of an electrical contact member with a first force normal to the second surface while the portion of the wire is vibrated with ultrasonic frequency along the second surface by means of a suitable tool in order to generate a bond between the portion of the wire and the electrical contact member by welding. According to the method the time dependent deformation of the wire and the time dependent vibration amplitude of the tool along the second surface are measured during generation of the bond and compared separately to corresponding master curves, so as to judge the stability of a generated bond. In case the difference between the respective master curve and the corresponding measured quantity surpasses a predefined tolerance value, the respective bond is considered to be a deficient bond.